1. Field of the Invention
The present invention relates to an imaging apparatus using an imaging device such as charge coupled device (CCD) or the like
2. Description of Related Art
There have been known a variety of semiconductor imaging devices for transforming an optical image formed by an optical system into an electrical image signal. Examples of these devices include CCD and MOS-type image sensors. Conventional semiconductor imaging devices known generally are fabricated on a semiconductor substrate, in which are included a plurality of photoelectric conversion elements and a signal read circuit that reads out charges of images created by the photoelectric conversion elements. The photoelectric conversion elements yield charges in response to light from an object to be imaged, and charges of the photoelectric conversion elements are read out sequentially by the signal read circuit and transferred to an output terminal, from which an image signal is led out. These conventional imaging devices have been used widely in imaging apparatus, in which cases optical images formed by the visible light are rendered into electrical image processings. The conventional semiconductor imaging devices are all intended to record continuous optical information or optical information that varies at an interval up to the period of imaging (one image-field period, e.g., 1/30 second), and they are not suitable for recording pulse-wise optical information having a shorter duration and a longer period than the imaging period of time.
For the observation of the structure of various substances including biological tissues with a microscope, the spatial resolution can be upgraded by using an illumination light with a shorter wavelength. On this account, a pulsed X-ray that is generated when a laser beam is projected to the target metal is used for the illumination light, and a resolution of 1 .mu.m or less is achieved at present.
Conventionally, an observation image produced by the above-mentioned X-ray microscope is recorded on a light-sensitive film by multiple X-ray pulses in a long exposure time, and the exposed film is processed for development. This is very lengthly, delicate and tedious work. Moreover, when the recorded observation image on the film is rendered the electronic image processing for the improvement of image quality or the implementation of various analyses, the developed photographic image needs to be converted into electronic information. This imposes a time gap between the optical observation and image data processing.
As a conceivable solution of the foregoing matter, when a semiconductor imaging device based on CCD or the like is used in place of the photographic film for recording the observation image, the conventional imaging devices encounter the following problems.
The X-ray microscope has an X-ray emission period of about 1/10 second for illumination of an observation object, which is determined from the retention of thermal stability of the pulse laser source, whereas the semiconductor imaging device has an imaging period as short as about 1/30 second. Therefore, when the X-ray microscope is used with the conventional imaging device, it produces images of the illuminated object for some periods among consecutive imaging periods and produces dark images without illumination for the remaining periods. Namely, the brightness of images varies along imaging periods (individual image fields), resulting in an extremely deteriorated image quality. This impropriety is not specific to the recording of observation images of the X-ray microscope, but is a common problem of cases in which a visible or invisible pulsative optical image has a shorter duration and longer period than the imaging period of the imaging device.
The general conventional semiconductor imaging devices have their entire surfaces except for the photoelectric conversion elements, i.e., the surface of the signal read circuit including the vertical charge transfer elements and horizontal charge transfer elements, covered with a light-shielding film. The reason for the need of this light-shielding film for the conventional imaging devices is that if areas other than the photoelectric conversion elements are exposed to the light, noise charges created by it are mixed to signal charges, and the degraded S/N characteristics of the resulting image signal deteriorates the image quality. The signal read circuit may possibly not work when a large amount of light is incident to these areas. The imaging device includes a transfer gate circuit for transferring charges of the photoelectric conversion elements to the vertical charge transfer elements, and this circuit, which is formed of MOS transistors for example near the device surface, also needs to be covered with a light-shielding film.
However, the formation of the above-mentioned light-shielding film on the device surface encounters the following problems.
When a photoelectric conversion element, which is formed by a pn-Junction plane in the semiconductor substrate, is covered at its periphery with a light-shielding film, part of the film penetrates into the pn-junction area, and part of the photoelectric conversion element at the edge of the pn-junction area may possibly not work. This means that the pn-junction plane which works as the photoelectric conversion element has its opening ratio reduced, resulting in a degraded sensitivity of the device.
The manufacturing of imaging devices with the formation of the light-shielding film often suffers a poor yield, and an investigation has attributed most defects to the short-circuit between the light-shielding film and the gate electrode and between the light-shielding film and the metallic wiring.
In addition, the light-shielding film for the conventional semiconductor imaging devices is formed of an aluminum thin film with a thickness of about 1 .mu.m. Although this film shields the visible light completely, it is insufficient against some invisible lights. For example, this film transmits 30% or more of X-rays with a 20 angstrom wavelength. If the film thickness is increased to 7 .mu.m, the transmittance of X-rays can be reduced to 0.02%. However, the range of pn-junction plane that does not work for the photoelectric conversion element increases to several microns, and the resulting extremely small opening ratio of the pn-junction plane will end up providing the device with an extremely low sensitivity.